Nobz (and Boss Nobz) "replace its slugga and choppa with items from the
Nob Weapons list."
That means you lose both the Slugga and Choppa, then follow the rules on the
Nob Weapons list of what weapons you may select from the list.
So, if you want just a Big Choppa, you replace both the Slugga and Choppa for a Big Choppa from the list.
If you want a Big Choppa and a Slugga, you replace both the Slugga and Choppa for a Big Choppa and Slugga from the list.
While you technically have to swap both weapons out to pick from the list, because both the Choppa and Slugga are on the "take two" portion of the list, you can in practice "swap out" just the Slugga or Choppa for any weapon on the list.